Former Sun Microsystems chief strategy officer William J. Raduchel, 53, will take over the role, AOL said. Andreessen, 28, who joined AOL in February after the acquisition of Netscape, will stay on as a part-time "strategic adviser" focusing on emerging technologies and investments. Raduchel will join AOL on September 13.
"Marc wanted to spend more time working with start-ups, and we are pleased that this arrangement will allow him to do that in a manner that furthers our investment objectives," AOL chief executive Steve Case said in a statement.
Andreessen was the first chief technology officer for AOL and was hired with the directive to help the company spot new trends and technology it could use to improve its products.
"In general I'm disappointed--seems like all of the senior people have left Netscape," said Michael Cusumano, a professor at the Sloan School of Management at the Massachusetts Institute of Technology. He is the author of Competing on Internet Time, a book that chronicled Netscape's life and times.
Among the senior Netscape executives who have left are former chief executive Jim Barksdale, former chief administrative officer Peter Currie, former investor relations and new business development manager Quincy Smith, former global public policy counsel Peter Harter, former senior vice president of business development Jennifer Bailey, and former general counsel Roberta Katz.
"The AOL culture is very different from Netscape," Cusumano added.
AOL is gaining a seasoned executive with Raduchel, who joined Sun in 1988 as vice president of corporate development and planning. He served as chief information officer from 1990 to 1998.
Raduchel was a chief negotiator in Sun Microsystems' alliance with America Online and Netscape, unveiled in November when AOL said it would purchase Netscape. Sun said it isn't sure whether it will replace Raduchel.
Andreessen last month filed to sell 940,680 AOL shares, worth $88 million, according to a filing with the Securities and Exchange Commission. He still holds a large stake in the company.
The move comes as AOL is locked in an ongoing fight with Microsoft and others over popular instant messaging technology. AOL enjoys significant market share in instant messaging because of its AOL Instant Messenger (AIM) and ICQ products--but that share is threatened as Microsoft has turned its focus on the technology.
When Microsoft launched its MSN Messenger product in July, it let users communicate with AIM users after they entered their AIM password. AOL accused Microsoft of hacking into its servers to offer the feature.
AOL has been blocking Microsoft's access to its AIM users and has automatically logged off Messenger users trying to chat with AOL users. Microsoft in return has posted fixes to skirt around the blockades.
As an undergraduate in 1992 at the University of Illinois National Center for Supercomputer Applications, Andreessen and his less-celebrated partner Eric Bina wrote Mosaic, acknowledged as the first Web browser, in only six weeks.
Andreessen later moved to Silicon Valley to work for Enterprise Integration Technology, a small software company. An email from Jim Clark, cofounder of Silicon Graphics, led to a series of meetings that ultimately would spawn Mosaic Communications--later renamed Netscape Communications--in the spring of 1994.
News.com's Mike Ricciuti and Bloomberg contributed to this report.